In multiple-track data-storage apparatus of the type in which the transducer head is stepped or otherwise moved on command to any of a series of parallel recording tracks on a record member, for example magnetic tape drives used in computer systems for data back-up and archival storage purposes, a variety of different positioning mechanisms are or may potentially be used. Most such systems employ stepper motors, which are very accurately controllable since they produce a particular and specific amount of output shaft rotation as a function of the number of excitation pulses applied, and such pulse actuation lends itself to very accurate control.
Where the physical arrangement and environment permits, the stepper motors in such head-positioning systems are typically located in an area directly adjacent the place where the transducer head is to be located in order to access the recording media, since it is desirable to move the transducer head directly from the output shaft of the stepper motor because this allows for very accurate and consistent head-positioning. Where the architecture (physical arrangement of parts) for the drive is more confined, however, as is frequently the case in present-day miniaturized streaming cartridge-type magnetic tape drives, far more exacting conditions of physical confinement exist since the amount of available space is extremely limited; thus, the head-positioning motor must sometimes be located at a considerable distance from the location of the head itself. In such situations, an elongated linkage of one type or another must be utilized to translate motion from the head-positioning motor to the head, which is normally mounted in or on some head-carrier having a guide system which ensures the necessary accuracy in lateral movement of the head across the recording media. Use of such elongated motion-transfer apparatus inevitably tends to introduce error in the resulting positioning of the head, due to a number of reasons.
One successful previous system for use in such applications is shown in U.S. Pat. No. 4,647,994, which is commonly owned with the present patent (and which should be considered incorporated by reference herein). In that system, a miniaturized cartridge-type tape drive is shown which lends itself to implementation in a very small physical volume and yet which provides for accurate positioning of the transducer head at a plurality of locations with respect to the recording media (i.e., tape). In this system, the head-positioning motor is actually located in the rearward-most extremity of the tape drive, due to the extremely demanding physical constraints presented by the very limited packaging envelope, and due to the fact that direct drive of the tape-transporting capstan is considered desirable, and is achieved, by placement of the capstan-drive motor in the only location which is available close to the point where the transducer head must of necessity be located. Accordingly, in the system shown in such prior patent, a pivotal arm system is utilized to translate motion from a rotary cam driven by the head-positioning motor forward through the frame and other componentry of the drive and up to the point where the head is located. Such a system does in fact work well under many conditions, and has been successfully used for a number of years in commercially successful data-storage tape drives which utilize a number of comparatively narrow, closely-spaced recording tracks (for example, eight to twelve different tracks extending parallel to one another across the width of recording tape, which is nominally one-quarter inch wide).
Under circumstances encountered at the present point in time, however, track densities far more demanding than those just noted are becoming required, to increase the available storage capacity on the media (tape). Thus, one may now contemplate the use of many more such tracks on the same width of tape, each of which is narrower than those utilized in the past and far more closely spaced to adjacent tracks. Consequently, a pivotal head-positioning arm such as that shown in U.S. Pat. No. 4,647,994 ultimately reaches its maximum performance limit, since while the stepper motor which initiates changes in head position may be accurately controlled, and while mechanisms are available by which the carefully-controlled stepper motor shaft rotation may be consistently and accurately outputted to the positioning system, it is inevitably true that such a pivotal arm will not in fact move the head exactly the same distance for each step or group of steps initiated by the positioning motor, inasmuch as the ends of the pivotal arm move on a radius over an arcuate path. Thus, depending upon the particular dimensions of the pivotal arm and the relative position of its pivot axis with respect to that of the head and recording media, a given increment of pivot arm rotation will produce a different and varying amount of ensuing head movement at each different position which the arm may occupy during its operation. This produces track spacing variations which, while unimportant under prior state-of-the-art conditions, becomes extremely important where it is desired to use substantially the entire width of the media with very closely-spaced adjacent tracks, without wasting any available space on the media; of course, the problem becomes even more exacerbated where the recording tracks are extremely narrow, as is true of the present state-of-the-art.